Hurtling over the Martian Surface Phobos is a Unique Satellite and the Largest Martian Moon

Phobos – the name means “fear” in Greek after one of the sons of the War God Ares or Mars as the planet was known to the Romans. Phobos is the larger of the two known satellites of the red planet. It was discovered by Asaph Hall, the chief of the United States Naval Observatory in Washington D.C. on August 8, 1877. Hall’s discovery is all the more impressive in that the self educated mathematician and astronomer found Phobos barely a week after finding the smaller moon, Deimos, and long after most of the scientific community had decided that Mars in fact had no satellites. 

Phobos is a particularly interesting satellite. Its orbit is less than 10,000 kilometers from the center of Mars, meaning that Phobos practically scrapes the surface of its planet at a distance of less than 6000 kilometers. To put this in perspective Phobos is about 45 times closer to Martian soil than is the moon to Earth, and closer to its planet than any other known moon.  Phobos is not a large satellite; it has a mean circumference of just 69.7 Kilometers and a surface area of only 1548 square km. Its gravity is negligible. 

Phobos orbits Mars so closely that it rockets over the planet, passing from west to east as observed from the Martian surface in under 4 hours, 15 minutes, and completing more than 3 orbits on average per earth day. As Phobos is orbiting faster than Mars rotates on its axis it is gradually spiraling in towards the Martian surface. Best estimates are that when Phobos finally descends to about 7000 kilometers from the center of Mars tidal forces will tear the satellite apart and the debris will form a ring superficially similar to those of Saturn. 

For a long time it was assumed that this would take perhaps 50 million years but more recent researchand calculations seem to indicate that Phobos has perhaps 10 million years at most before it is reduced to fragments. 

Phobos and her sister moon Deimos are composed of materials far different than that of the Martian surface and are generally considered to be captured class C asteroids; their structure is that of a ball of rubble with spaces between the materials that account for a lighter mass than a solid body of the same size would be expected to possess. In the case of Phobos the difference is about 30 %, give or take a fraction of a percent. 

The features that make Phobos so interesting have also made it the object of radical speculations, which have at times been treated with seriousness in the popular press. The low orbit, which is not easy to explain in terms of celestial mechanics and its unusually light mass for a body of its size lead Soviet astrophysicist Iosif Shklovsky to speculate that only a hollow body could perform in this way, and since a hollow body of this magnitude is not known to occur in nature, then Phobos must in fact be an artificial structure and therefore evidence of a vastly superior civilization. This was apparently given at least some credibility by the Eisenhower White House and lives on in UFO lore and apparently, a fringe of the scientific community today. 

Close flybys by robotic mars discovery vehicles, notably Viking, have quashed this theory and Phobos appears to be what most of the scientific world predicted that it would be, a captured impact cratered Class C asteroid. 

Phobos remains however an object of intense interest and it is a tragedy, in terms of what now will not be discovered that the 2011 Russian sponsored Phobos – grunt mission to Phobos failed to respond to ground control command and will never be able to rendezvous with the little Martian moon. It can only be hoped that a slashed NASA budget and global recession will not close the door to further investigation of this most interesting of satellites, the anomalous ball of rock that orbits Earths nearest planetary neighbor. The true story of Phobos might prove to be even stranger than the wildest speculations about it that persist to this day.